*Bruce W. Downing, M.Sc., P.Geo. is Senior Geologist, Gamah International Ltd., Vancouver.

**John Gravel, M.Sc., P.Geo. is Quality System Manager, Acme Analytical Laboratories Ltd., Vancouver.

 

INTRODUCTION

Acid rock drainage (ARD) has an impact upon our water, soil and plant quality and fish habitat with regard to trace elements, whether in a toxic or non-toxic, available or non-available form. Its impact will be determined by the type and amount of trace element(s) within the host medium (e.g. mineral assemblage). Prediction of trace element geochemistry is readily determined from standard analytical methods, providing that the sample medium is collected and analyzed in a manner that includes a program of Quality Assurance and Quality Control (QA/QC). The cost of trace element analyses and geochemistry prediction is relatively inexpensive when undertaken as part of an overall ARD study.

Metal leaching is a naturally occurring process. Under certain conditions if the trace elements in the original media are in anomalously high concentrations they may have a negative impact on the receiving environment. Metal leaching is generally associated with acid rock drainage, however it can occur under non ARD conditions. It is thus imperative that trace element geochemistry be a major part of any ARD study, as it is generally site-specific. Metal leaching and acid rock drainage policy and guidelines are becoming standard practice for most government environmental agencies. It should be up to the proponent to help set the receiving environment objectives and permitting conditions. Interpretations from the results of an ARD study may be used to predict metal leaching potential, however the time scale of metal leaching commencement, duration and termination cannot be determined with certainty from laboratory testwork. A review of geochemical software models devised for prediction of acidic drainage do not really address the process of metal leaching (Perkins et al., 1995).

Trace elements in a geological context are defined as:

"all elements except the eight abundant rock-forming elements: oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium" (Thrush et al., 1968).

The terms minor elements and trace elements are considered to be synonymous. Trace elements may be grouped site specifically by concentration levels, such as ore-mineral elements (Cu in chalcopyrite, Zn in sphalerite, Ni in pentlandite, etc.), ore mineral trace elements (Cd in sphalerite) or non-ore mineral trace elements (Cu, Co and Ni in pyrite). Trace elements are often present at concentrations below the lower limit of detection of analytical procedures.

In a study of weatherability of sulphides by Kwong (1993), the effect of trace elements in pyrite was included . His preliminary investigation concluded that

"the incorporation of both interstitial impurities and mineral inclusions in pyrite will cause local strain in the crystal structure, rendering the pyrite more susceptible to alteration"

and that little information is known regarding trace element content on pyrite weatherability .

The identification of problematic trace elements should be done at an early stage in both the exploration and feasibility stages phases of a project. The costs of treating the problematic trace element(s) can be very high, but if the source(s) can be identified, then its selective treatment may be more cost effective than the treatment of a much large quantity of non problematic material with which it is arbitrarily classified (Wilson, 1993). It is imperative that trace element geochemistry be done of potential waste rock that may considered for blending. Blending may be a remediation for neutralization, but it may have some severe impact upon trace element (metal) leaching if the waste rock is not characterized properly.

The two most common sulphides that generate ARD are pyrite and pyrrhotite. Trace elements in both these minerals can vary from site to site, and from location to location within a site (Hawley and Nichol, 1961). Trace elements of pyrite has been investigated and reported by Uytenbogaardt & Burke, 1971, Auger, 1941, Bjřrlykke, 1945, 1947, Fleischer, 1955 and Sztrókay, 1944. It is essential that any determinations made on pyrite or other sulphide mineral samples be done on well defined and specified samples such as sections of diamond drill core and/or specific locations from a mineral or mine property. Studies by Kwong and Whitely (1993) indicate that trace heavy elements may have an impact upon the weatherability of sulphides and hence the rate of ARD generation

Trace element geochemistry should not be just limited to acid rock generation sites, but applied to any project that involves creating, removal and storage of waste material such as rock, soil and tailings. This would include industrial minerals such as diamonds and coal (Goodarzi, 1994). Mineral deposits are unique in that each type of deposit will have a particular geochemical signature. Explorationists use trace element geochemistry to locate these deposits through analysis of soil, water, rock and plant material (Roberts, 1982; Ryall, 1977). The tectonic environment of formation of volcanogenic massive sulphide deposits and porphyry tin and copper-molybdenum deposits can be identified from the geochemical characteristics of the associated igneous rocks (Pearce & Gale,1975). The incorporation of trace elements into minerals occurs at the time of formation but can be altered over geologic time with changes in temperature, pressure and alteration resulting from hydrothermal fluids (Raiswell & Plant, 1980 and Madeisky, 1995). A plot of Mo versus depth from a drill hole which intersected disseminated nickel-bearing sulphides in dunite shows the influence of hydrothermal activity as indicated by the elevated Mo values (Figure 1).

Figure 1: Variation of Molybdenum Concentration with Depth for Diamond Drill Hole in Nickeliferous Dunite

Metal mineralization may occur as disseminations in host rock, in veins, as massive sulphides and fracture fillings. A deposit may contain one or several of these types of metal mineralization. Each type may either have different metals and trace elements or varying concentrations of the same. This variation will certainly have an impact upon waste material characterization and disposal.

METHODOLOGY

Sampling

Sampling is a major part of the program, and has been discussed on the Quality Assurance/Quality Control page. Inadequate or inconsistent sampling procedures produce data of questionable quality which may lead to erroneous interpretation and conclusions.

Analytical Techniques

Most analytical testing constitutes three steps, namely sample preparation, decomposition and determination.

Preparation

Sample representivity is requisite to good geochemical analysis. Following proper sample collection (Downing and Mills, 1998) that ensures adequate characterization of the geological entity, preparation must reduce the sample volume to a size suitable for analysis yet preserves the bulk geochemical signature of the larger body. Realistically, a mass of several tens of kilograms to several tens of thousands of tonnes may be defined by an analytical test on a 500 mg sub-sample. Using Gy’s theorem (Gy, 1963, 1965, Ottley, 1966) and some foreknowledge of the sample material, the analyst can calculate the volume of sub-sample and grain size needed for a statistically valid chemical analysis. Sample reduction entails comminution by sieving or crushing and grinding. Standard procedure at most laboratories is to sieve soils and sediments to minus 80 mesh ASTM (minus 177 µm). Rocks are crushed to minus 10 Mesh ASTM (minus 1.000 mm) then a riffle split sub-sample is pulverized to minus 150 Mesh ASTM (minus 100 µm) or finer. A substantially larger sub-sample comprising a wide range of grain sizes may be employed if the focus of investigation is trace elements residing as coatings on mineral grains. Water samples generally do not require preparation other than to re-suspend any settled material by agitating the sample prior to analysis.

Point source geochemical analysis requires segregating minerals of interest. Typically these have a higher density so gravity separation using heavy liquids (see Mills, 1978, 1986) or mechanical jigs (see Burt and Mills, 1984) is used. The resulting concentrate is then "picked" by a trained mineralogist for the desired mineral(s).

Decomposition

Sample decomposition can be total, partial or selective given the requirements of analysis. Fusion with fluxes such as sodium peroxide-sodium carbonate or mixtures of strong mineral acids such as hydrofluoric-perchloric-nitric-hydrochloric acids will digest all minerals within the sample thereby permitting total determination of trace elements. Strong acids used singularly or in certain combinations will dissolve some minerals while leaving others intact for a partial determination of trace elements. For example, nitric acid used alone or in combination with hydrochloric acid (aqua regia) will decompose sulphides and most metal oxides, hydroxides and carbonates. With either total or partial decomposition, the investigator can determine the total potential load of trace metals in the environment. Liberation of the trace metals may be studied with a selective or sequential digestion scheme. A sample can be sequentially leached with distilled water, ion exchange reagents, oxidizing or reducing agents and weak or dilute acids and bases to progressively strip away trace metals residing in various substrates (Chao, 1984). The investigator can thus quantify bioavailability or ascertain the rate of loading of trace metals in the environment.

Determination

State of the art multi-element analyses are instrumental determinations measuring mass, light emission or gamma radiation properties of atoms within the sample.

Inductively coupled plasma emission spectrography (ICP-ES) delivering concentrations for 30 to 40 different elements is a common geochemical analysis tool. Sample solutions are aspirated into a plasma operating at 8000 °K. Light emitted by excited atoms returning to the ground state is split into its spectral wavelengths by an Eschelle grating. The intensities of the various wavelengths is measured by photomultiplier tubes or - in the most modern ICP-ES models - by a CCD chip. Linear range is five orders of magnitude with detection limits in the ppm to ppb level.

Inductively coupled plasma mass spectrography (ICP-MS) capable of quantifying 60 to 70 elements is fast becoming the instrument of choice for geochemical analysis. Like the ICP-ES, a sample solution is aspirated into a high temperature plasma to generate ions. The ions pass through a magnetic quadrapole that deflects their flight path, with the degree of deflection related to the mass of each ion. A sampler measures the number of atoms defected under a given magnetic field specific to a certain element to determine concentration. Linear range is six to seven orders of magnitude with detection limits in the ppb to ppt level. The instrument is best applied to analysis of waters, effluents and solutions with low concentrations of dissolved solids.

Instrumental neutron activation analysis is a non-destructive technique for the determination of 35 or more elements including Au. The method is particularly well suited for the analysis of rare earths (La, Ce, Nd, etc.), incompatible elements (Hf, Nb, Ta) , some trace (As, Co, Sb), minor (Ba, Rb) and major (Fe, Mg, Na) elements. However, the method is inadequate for base metals (Cu, Pb and Zn). Sample powders are encapsulated and exposed to the neutron flux in the heart of a nuclear reactor. The samples are removed and allowed to cool for several days to permit for decay of the more active gamma ray emitters. Subsequently the samples are placed in a gamma ray detector to determine element concentrations by measuring the intensities of the various gamma ray wavelengths. Linear range is 5 orders of magnitude with detection limits in the ppm to ppb levels.

Analyses can either be single source or bulk. A single source would consist of a single mineral or mineral grain, while bulk would consist of the sample as a whole, which may include several different mineral types.

Bulk Source Analysis

Rock and soil trace element analyses are generally obtained from a 30 element Inductively Coupled Plasma (ICP) analysis with an aqua regia digestion. This digestion, however, gives only a partial leach for some elements. A total digestion would include a four acid dissolution procedure. Other analytical techniques may include 60 element ICP analysis, neutron activation, mobile metal ion analysis, enzyme leach, selective extractions,

Some trace elements can also be determined by X-ray fluorescence spectrophotometry (XRF), the method usually used for major oxide analysis. XRF is conducted on either pelletized or on fused samples, depending upon the level of detection required.

Water trace element analyses are generally performed by Inductively Coupled Plasma (ICP) analysis.

Single Source Analysis

This analysis is limited to specific mineral (or mineral grains), and is generally carried out using petrographic and mineralogic examination using transmitted and reflected light microscopy, and by various X-ray diffraction (XRD) techniques. Although electron probe microanalysis (EPMA), scanning electron microscopy (SEM) and other more specialized techniques are employed, their use is generally confined to sulphide minerals where compositional abnormalities affect ARD testwork interpretation. Such techniques are particularly useful in the determination of the chemical composition of sulphide oxidation products such as rims, inclusions and amorphous (non-crystalline) species..

An innovative method, as proposed by Mills and Downing (1994) and comprising three sub-procedures, is as follows:

1. Pyrite or other sulphide isolation

a. Crushing and partial grinding of samples to minus 850 mm.

b. Dry screening of samples to minus 850 mm plus 90 mm (this range will depend upon mineral liberation characteristics)

c. Removal of highly magnetically susceptible minerals (magnetite) by low intensity magnetic separation.

d. Isolation of specific sulphide minerals by Relative Density separation with a Magstream® 50 gravity separator (Domenico et al., 1994). Such a unit can make split-points at Relative Densities from 1.5 to 18.0 with a precision of ±0.09 Relative Density.

The Relative Densities of some relevant sulphide minerals are given below in Table 1- values are taken from Jones and Fleming (1965) and Tillé and Panou (1965).

Table 1: Sulphide Mineral Relative Densities (from Jones and Fleming (1965) and Tillé and Panou (1965)

 

RELATIVE DENSITY

MINERAL

JONES & FLEMING

TILLÉ & PANOU

Arsenopyrite

5.9-6.2

-

Chalcopyrite

4.1-4.3

4.1-4.3

Galena

7.4-7.6

7.5-7.6

Pyrite

5.0-5.1

4.8-5.0

Pyrrhotite

4.6

4.6-4.75

Sphalerite

3.9-4.1

3.9-4.1

Isolation of the above sulphides from gangue minerals or from each other are extremely difficult, if not impossible using conventional heavy liquid techniques (see Mills, 1978, 1986), even with the use of Clerici Solution at elevated temperatures. The Magstream® separator, however, is capable of such separations. Domenico et al. (1994) describe the extensive use of a Magstream® unit in comparison with conventional heavy liquid methods using halogenated hydrocarbons in a program involving 70,000 samples. The Magstream® laboratory unit treated 90 samples per day with a lower sample cost in terms of chemicals used (ferrofluid versus halogenated hydrocarbon). The ferrofluids used by the Magstream® are also non-toxic when compared with the highly toxic halogenated hydrocarbon. Results obtained with the Magstream® and heavy liquids were comparable.

Each sample concentrate should be examined using a binocular microscope and/or analysis by X-Ray Diffraction (XRD) to ensure sample purity.

It may be necessary in some cases to enhance mineral isolation using a Frantz Isodynamic® separator (McAndrew, 1957) to treat Relative Density fractions.

2. Chemical analysis of isolated mineral samples by ICPMS.

3. Oxidation rate studies on isolated sulphide mineral samples, using the method developed by Klein et al. (1994) to simulate natural oxidation, and chemical analysis of aqueous leachate to determine soluble trace elements. The purpose of this is to determine relative oxidation rates for various sample sulphide minerals and to establish the relative solubilization of trace elements from these various samples under natural conditions .

This procedure will provide insight into the degree of solubilization of trace elements during natural sulphide mineral oxidation, and the overall results should aid in the prediction of acid mine drainage potential and generation rate, possibly indicating the type of routine test that may be suitable for future use in AMD prediction.

Data Analysis

The methods for determination of natural and elevated to toxic concentrations of both dissolved and non-dissolved components in various medium lends itself to both statistical and elemental plotting techniques.

Runnells et al. (1998) have shown that probability plots can be used to determine natural background concentrations of dissolved components in water at mining, milling and smelting sites.

CASE HISTORIES

Former Britannia Mine, British Columbia

This mine was operated from 1902 to 1963 by the Britannia Mining and Smelting Company Ltd., and from 1963 to 1974 by Anaconda Mining Company. Mining activity ceased in 1974 because of the exhaustion of economic ore. Because of an environmental concern for ARD (see Former Britannia Mine, Mount Sheer - Britannia Beach, B.C. page) water quality monitoring has continued into the late 90s. Most of the underground workings of the mine are currently relatively inaccessible, and representative rock sampling is not possible. However, Table 2 shows some major, minor and trace elemental analyses for three tailings samples and one ore sample from the former Britannia Mine during production. These analyses are reported in DBARD3, the British Columbia Database for Acid Rock Drainage (Lawrence and Harries, 1996), but their source and history is not known.

Table 2: Major, Minor and Trace Elemental Analyses for Four Rock Samples from the Former Britannia Copper Mine (DBARD3, Lawrence and Harries, 1996)

  SAMPLE NAME AND NUMBER
  TAILINGS ORE TAILINGS TAILINGS
ELEMENT 880355-071 C6 880355-072 880355-076
Al, % 2.68 4.44 7.17 7.44
As, ppm 3450 80    
Ag, ppm     0.2  
B, ppm        
Ba, ppm 70 2130 90 90
Ca, % 4.15 0.4 3.74 3.19
Cd, ppm 1 15 2 2.5
Co, ppm 36.9 34 16 11
Cr, ppm 37.7 8 54 56
Cu, ppm 3450   196 75
Fe, % 9.06   5.78 5.16
Hg, ppm        
K, %     0.34 0.36
Li, ppm        
Mg, % 2.33 2.9 1.62 1.69
Mn, ppm 1260 96 1235 1060
Mo, ppm   8 3 1
Na, % 0.052 0.2 3.99 4.19
Ni, ppm 19 30 11 8
Pb, ppm 80 400 48 14
Sb, ppm        
Se, ppm        
Si, % 0.341 1.6    
Sn, ppm   20    
Sr, ppm 155 40 162 147
V, ppm 267 30 248 247
Zn, ppm 272 2610 78 72

The elements present in the ore and tailings samples may be divided into four groups, depending upon their concentration:

  • Major rock-forming mineral elements: Al, Ca, Fe, K, Mg, Mn, Na, Si.

  • Major ore-mineral elements, As (1 sample, probably tennantite), Cu, Fe, Zn (chalcopyrite, pyrite, sphalerite).

  • Trace elements, Ag (argentite), Ba (barite), Cd, Co, Cr, Mo, Ni. Pb (galena), Sn, Sr (celestite), V.

  • At or below detection limit or not determined: B, Hg, Li, Sb, Se.

Table 3 shows Acid Base Accounting (ABA) properties for the three tailings samples.

Table 3: Acid Base Accounting (ABA) Properties for Three Rock Samples from the Former Britannia Copper Mine (DBARD3, Lawrence and Harries, 1996)

 

SAMPLE NAME AND NUMBER

 

TAILINGS

TAILINGS

TAILINGS

ABA PROPERTY

880355-071

880355-072

880355-076

Sulphide S, %

1.3

1.26

2.25

AP, kg/tonne CaCO3

40.6

39.3

70.3

NP (Sobek), kg/tonne CaCO3

10.8

10.3

13.7

NPR (NP/AP)

0.26

0.22

0.19

With NPR values of 0.26, 0.22 and 0.19, all three of the tailings samples are considered to be potentially acid generating (PAG).

Figure 2 shows total metal concentrations and pH for the 2200 Level Portal discharge (ARD) at the Britannia Mine for a 17 week period during 1995.

Figure 2: Major, Minor and Trace Element Monitoring Data, 2200 Level Adit Effluent (ARD), Britannia Mine, BC - 1995 (Data courtesy of Gordon Ford, BC Ministry of Environment)

The elements present in the 2200 Level effluent (ARD) may be divided into four groups, depending upon their concentration

  • Major elements, >10 mg/l: Al, Ca, Cu, Fe, Mg, Si, Zn

  • Minor elements, <10>0.1 mg/l: Bi, Cd, Mn, Sr

  • Trace elements, <0.1 mg/l: As, B, Ba, Co, Ni, Pb, Sn, Ti

  • At or below detection limit: Ag, Be, Cr, Mo, Sb, Se, Te, Tl, V, Zr

Although the actual tailings analyzed and reported in Tables 2 and 3 are not claimed to be representative of the rock source of the ARD discharging from the 2200 Level (Figure 1), they were produced from the Britannia mine. The trace elements Ba, Cd, Co, Ni, Pb and Sr in the rock (tailings) samples all appear as trace or minor elements in the ARD. The major rock forming mineral elements Al, Ca, Fe, Mg, and Si are major elements in the ARD indicating the probable dissolution of calcite, dolomite and aluminosilicates (K and Na analyses were not conducted on the ARD), and Mn is present as a minor element. The major ore mineral elements Cu, Fe and Zn are all major elements in the ARD. As appears as a trace element in the ARD (although it was reported as below detection limits for more than 50% of the samples), but was reported for only one of the samples.

In general terms the above discussion demonstrates that trace metal concentrations in ARD can be predicted qualitatively, and possibly semi-quantitatively, from the chemical and mineralogical analyses of host rock and ore. A more detailed analysis, including that of individual sulphide mineral grains, would indicate trace element-mineral associations such as Ni and Co with pyrite and/or chalcopyrite, and Cd with sphalerite.

 

A Disseminated Nickel-Cobalt Sulphide Deposit

The following data is from a disseminated nickel-cobalt sulphide deposit hosted by a serpentinized ultramafic complex. The predominate host rock is dunite. Diamond drilling has indicated widespread mineralization extending up 490 metres in depth. Drill core was sampled over two metre intervals and analyzed for nickel by standard assay method and 30 elements by ICP-MS using an aqua regia digestion. The purpose of the 30 element analysis is to map trace elements over the drill hole length and deposit as a whole. The aqua regia digestion is essentially a partial leach for silicates, but a complete leach for to carbonates and sulphides. Plots of Ni, Co, Cu, Cr, Mo and Fe versus depth are presented in Figure 3.

Figure 3: Metal Concentrations versus Depth for a Disseminated Nickel-Cobalt Sulphide Deposit

Nickel: Plot shows distribution of varying nickel grades. This plot will indicate the sections of potential waste rock from proposed cut-off grades of 0.2% and 0.4 % nickel.

Molybdenum: Plot shows distribution of molybdenum values. This plot indicates the section of different sulphide mineralization due to hydrothermal influence on the host rock.

Copper: Plot shows distribution of copper values. This plot indicates the background level of copper in potential waste rock.

Chromium: Plot shows distribution of chromium values. This plot indicates the level of chromium in potential ore and waste rock. Chromium mineralization is a function of the original pressure, temperature, sulphur and oxygen conditions at time of formation of the ultramafic. The background level for chromium would be in excess of 500 ppm, and thus would be apparent in water geochemistry.

Iron: Plot shows distribution of varying iron values. This plot (in conjunction with the drill logs) indicates the sections of different types of sulphide mineralization. The higher Fe values coincide with high nickel values. It should also be noted that the Fe values are due to both sulphide and silicate minerals.

In general, Figure 3 indicates that copper and cobalt are associated with the iron-nickel sulphide mineralization (predominantly pentlandite, (Fe,Ni)9S8 ), at surface to 30 m (hydrothermal activity), 55-75 m and 200-240 m, whereas chromium is not. Higher molybdenum values are predominantly associated with the hydrothermal activity between surface and 30 m. The generation of ARD from the sulphide mineralization would thus tend to mobilize both copper and cobalt (in addition to iron and nickel), and possibly molybdenum, but not chromium.

Trace element plots should be used to show the distribution of trace elements that occur both with varying lithologies, ore and non-ore grade material and spatially within a deposit. It should be noted that it is not sufficient to calculate an average or median value for each trace element due to the potential variability within a deposit.

 

Trace Elements in Copper Cliff Pyrrhotite

Multi-element X-ray maps can be extremely useful in the determination of trace element distribution within individual mineral grains. The following two X-ray maps (Shaw, 1996) show Fe, Ni, O and S distribution for a pyrrhotite grain (X-ray Map 1) and a pyrrhotite-pentlandite grain (X-ray Map 2) from Copper Cliff, Ontario.

X-ray Map 1: Fe, Ni, O and S Distribution for Pyrrhotite Grain (Courtesy Shannon Shaw)

X-ray Map 2: Fe, Ni, O and S Distribution for Pyrrhotite-Pentlandite Grain (Courtesy Shannon Shaw)

 

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